GEK111712 Control & Protection

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gGEK 111712

June 2007

GE Energy

These instructions do not purport to cover all details or variations in equipment nor to provide for every possible contingency to be met in connection with installation, operation or maintenance. Should furtherinformation be desired or should particular problems arise which are not covered sufficiently for the purchaser's

purposes the matter should be referred to the GE Company.

© 2007 General Electric Company

Control and Protection Articles

DLN2.6+ Gas Fuel System





Control and Protection Articles DLN2.6+ Gas Fuel System GEK 111712

3

TABLE OF CONTENTS

I. GENERAL ..................................................................................................................................................... 4 II. EQUIPMENT................................................................................................................................................. 4

A. Gas Flow Measurement (96FM-1)............................................................................................................ 4

B. Gas Fuel Auxiliary Stop Valve (VS4-1) ................................................................................................... 4 C. Stop/Speed Ratio Valve (VSR-1).............................................................................................................. 4 D. Gas Fuel Control Valves (VGC-1, VGC-2, VGC-3, VGC-4)................................................................... 5 E. Gas Purge Block Valves (VA13-1, VA13-2, VA13-20, VA13-21, VA13-3, VA13-4, VA13-23, VA12-

24) ............................................................................................................................................................. 5 F. Gas Purge Pressure Ratio Monitoring (96GN-1, 96GN-2, 96GN-3, 96GN-4) ......................................... 5

III. OPERATION................................................................................................................................................. 5 A. General ...................................................................................................................................................... 5 B. Pre-start Conditions................................................................................................................................... 7 C. Startup and Loading Operation ................................................................................................................. 8

IV. CONTROL................................................................................................................................................... 12 A. P2 Pressure Control................................................................................................................................. 12

B. Gas Flow and Split Control..................................................................................................................... 15 C. Gas Fuel Temperature Compensation..................................................................................................... 16 D. Gas Purge Control ................................................................................................................................... 17

V. PROTECTION ............................................................................................................................................ 17 A. P2 Pressure Control Protection and SRV Position Control Protection ................................................... 18 B. Fuel Temperature and Modified Wobbe Index Protection...................................................................... 18 C. Independent Flow Path Purge Protection................................................................................................ 19

LIST OF FIGURES

Figure 1. DLN2.6+ Fuel Nozzle Arrangement....................................................................................................... 6

Figure 2. Gas Fuel Leak Test Function Block Diagram....................................................................................... 10 Figure 3. SRV P2 Pressure Control Algorithm Diagram ..................................................................................... 13 Figure 4. SRV Speed Ratio Valve Control Schematic......................................................................................... 14 Figure 5. GCV Control Algorithm Diagram ........................................................................................................ 15 Figure 6. GCV Control Schematic ....................................................................................................................... 16

LIST OF TABLES

Table 1. DLN2.6+ Mode Staging Diagram............................................................................................................ 7 Table 2. Protective levels and actions for the Gas Fuel System – Gas Pressure.................................................. 18

Table 3. Independent Flow Path Purge Protection............................................................................................... 20



GEK 111712 Control and Protection Articles DLN2.6+ Gas Fuel System

4

I. GENERAL

The Gas Fuel System's function is to provide accurate and repeatable gas fuel flow, and flow split control

for a multi-stream fuel injection system DLN2.6+. It also provides safe and reliable isolation from the gas

fuel source. This unit use preheated Gas Fuel @ 365 deg F.

The DLN2.6+ control hardware and the control system are described in this document. See Appendix 1.

Simplified Schematic of the Gas Fuel System and Gas Purge System.

The Safer case ID for this System is EP-S-767.

II. EQUIPMENT

The Gas Fuel System consists of a gas flow metering tube, a gas fuel section of the accessory module (or

a separate gas fuel module for single shaft units), and the gas turbine on-base equipment. This section

houses both the gas fuel system, and gas fuel purge system. A brief description of the system’s overall

major components is described below. See Gas Fuel System Articles for further details.

A. Gas Flow Measurement (96FM-1)

A mass flow transmitter, 96FM-1 and Gas fuel meter tube / orifice (MG2-1) are used to measure

natural gas flow. The device, 96FM-1 operates on the principle of measuring delta pressure across the

gas fuel meter orifice, static gas pressure and gas temperature. These measured parameters establish

the mass flow when the flow area ratio (beta) is known (constant). Gas Fuel Flow is only used for

monitoring, and display in the gas turbine controls.

B. Gas Fuel Auxiliary Stop Valve (VS4-1)

The Auxiliary Stop Valve is a butterfly type isolation valve that provides the required ANSI Class VI

shutoff of the gas fuel supply. This valve is only required for units with performance heating of the

gas fuel. For units without performance heating, the Stop/Speed Ratio Valve (SRV) provides the

required ANSI Class VI shutoff capability and the Auxiliary Stop Valve is not required.

This valve is pneumatically actuated with a spring close actuator design for fail-safe operation. When

the system is pressurized or not tripped, the solenoid operated pilot valve (20VS4-1) directs

instrument air to the actuator of the aux. stop valve. The valve actuator acts against the valve spring

causing the valve to open. During a trip event the solenoid valve (20VS4-1) is de-energized which

vents the pressurized air in the Stop Valve actuator to atmosphere. The compressed spring causes the

Fuel Gas Auxiliary Stop valve to close. This valve opens during the gas turbine ignition sequence

when the turbine is started.

C. Stop/Speed Ratio Valve (VSR-1)

The SRV is a V-notch type ball valve that serves two functions. First it operates as the primary stopvalve, making it an integral part of the protection system. The SRV is tripped closed by the hydraulic

trip system via the directional trip relay VH5-1. This valve is hydraulically actuated with a spring

close actuator for fail-safe operation. An emergency trip or flame out on a normal shutdown will trip

the valve to its closed position, isolating gas fuel to the turbine. Closing the SRV can be achieved in

two ways: dumping the hydraulic trip oil to the SRV, or driving the SRV closed electrically using the

servo valve 90SR-1 with the control systems SRV position control loop. The other function of the

SRV is pressure regulation. The control system uses the SRV to regulate the pressure (P2) upstream

of the Gas Control Valves. This function is described in further detail in Section IV - Control.




5

D. Gas Fuel Control Valves (VGC-1, VGC-2, VGC-3, VGC-4)

There are four independent Gas Control Valves (GCV’s) in the DLN2.6+ system. The GCV’s are

angle body plug valves actuated by hydraulic cylinders with a spring close actuator for fail-safe

operation. The actuator design is single acting. The plugs in the GCV’s are contoured to provide a

proportional flow area in relation to valve stroke. The GCV’s use a skirted valve disc and venturi seat

to obtain high pressure recovery. This high pressure recovery design achieves critical pressure

operation at substantially lower valve pressure drops. The result is that the flow through the GCV’s is

independent of the pressure drop across the valves and is a function of valve inlet pressure (P2) and

valve area only. The valves are positioned by the control system to maintain a percentage of the total

fuel to each of the fuel circuits. This fuel split is a function of DLN operating mode, and the

combustion reference temperature.

E. Gas Purge Block Valves (VA13-1, VA13-2, VA13-20, VA13-21, VA13-3, VA13-4, VA13-23,

VA12-24)

The gas purge block valves are a V-notch type ball valve. These valves are arranged in a double

block and bleed configuration. The actuator design for each valve is a pneumatically operated rack-

and-pinion with a fail close spring. The valves are driven open and closed using a pneumatically

operated pilot valve (VA36-1, VA36-2, VA36-20, VA36-21, VA36-3, VA36-4, VA36-23, VA36-24). These valves provide for rapid exhaust of instrument air from the actuators of the VA13 valves.

The VA36 valves are driven by a solenoid operated pilot valve (20PG-1, 20PG-2, 20PG-20, 20PG-

21, 20PG-3, 20PG-4, 20PG-23, 20PG-24) for actuation. A metering needle valve is provided on the

inlet pressure side of the pilot valves (VA36) to control the opening rate of each VA13 valve, except

for VA13-2. This valve, VA13-2, is equipped with an I/P controller, used to provide for a variable

slew rate and positioning. The opening rate of the purge valves must be slow in order to minimize

transients. Excessive megawatt and temperature transients are caused by too rapid purging of gas fuel

from the fuel manifolds into the combustion system. The purge system is designed to minimize this

effect. Each valve has two limit switches that indicate open and closed valve position and are used for

monitoring and diagnostics in the control system.

F. Gas Purge Pressure Ratio Monitoring (96GN-1, 96GN-2, 96GN-3, 96GN-4)When a gas circuit is being purged, a minimum gas purge pressure ratio must be maintained to ensure

positive airflow across all the fuel nozzles. This pressure ratio is sufficient to overcome any

combustion can-to-can pressure variation. The differential pressure transmitters measure the gas

manifold pressure relative to compressor discharge pressure. These pressures are used for monitoring

and alarm in the control system.

III. OPERATION

A. General

Gas fuel flow is controlled with the Auxiliary Stop Valve, SRV, Diffusion GCV (D5) and the Premix

GCV’s (PM1, PM3 and PM2). The SRV and the GCV’s work in conjunction to regulate the total fuel

flow delivered to the gas turbine. The GCV’s control the desired fuel flow in response to a controlsystem fuel command, Fuel Stroke Reference (FSR). The response of the fuel flow to the GCV

position commands is made linear by maintaining a predetermined pressure upstream of the GCV’s

(P2). The GCV’s upstream P2 pressure is controlled by modulating the SRV as a function of turbine

percent speed (TNH), and feedback FPG2 from the P2 pressure transducers (96FG-2A, 96FG-2B,

96FG-2C).

The DLN2.6+ Gas Fuel System has four fuel manifolds (D5, PM1, PM3 & PM2). These are

independent fuel circuits each having an individual GCV for controlling gas fuel delivery. Each




6

combustion chamber has six fuel nozzles arranged in a five-around-one configuration. The PM1nozzle is in the center. The five outer nozzles have a diffusion passage and a premix passage (PM3 or

PM2). The center nozzle only receives premix fuel flow. Refer to Figure 1 for the DLN2.6+ Nozzle

Arrangement.

Circuit DescriptionD5 Outer nozzle diffusion passage

PM1 Center nozzle premix passage

PM3 Premix passage on 3 outer nozzles

PM2 Premix passage on 2 outer nozzles

Each of these four gas fuel circuits requires

continuous independent control. Thus, each has a

separate manifold and control valve.

PM1

D5

PM2

D5

PM3

D5

D5

PM3

D5

PM3

Figure 1. DLN2.6+ Fuel Nozzle Arrangement

Each fuel circuit requires a certain percentage of the total fuel. The percentage of fuel to each circuit

is a function of Combustion Reference Temperature (TTRF1) and DLN operating mode. There are

six steady state DLN modes of operation: Diffusion, Sub Piloted Premix, Piloted Premix, Sub

Premix, Premix and Extended Piloted Premix (not a normal operating mode). There are also two

transient load rejection modes. All fuel is directed to the PM1 nozzle during load rejections from

Premix or Sub-Premix modes. Load rejections from all other modes will be to Sub-Piloted Premix

mode. The steady state DLN operating modes are a function of Combustion Reference Temperature

(TTRF1). Refer to Table 1 DLN2.6+ Mode Staging Diagram.

This method of staging the fuel requires individual control valves (GCV-1, GCV-2, GCV-3, andGCV-4) for each fuel circuit. In order to optimize flow split accuracy, the control valves are operated

with a critical pressure ratio. A P2 pressure value, or control valve supply pressure, is

preprogrammed into the control scheme in order to maintain this critical pressure ratio throughout the

operating range. This control method ensures pressure fluctuations downstream of the gas valvesdoes not impact flow accuracy.

During certain DLN modes of operation, some circuits will have no fuel flow scheduled. A means of

purging these stagnant circuits is required to prevent condensate from accumulating, and to minimize

the potential for auto-ignition. The gas fuel that remains in a circuit after fuel flow is commanded off

is purged into the combustion chambers when the purge is commanded on. A connection to the purge

air system is located just downstream of the gas control valves. This ensures that the entire length of

pipe and manifold is purged.




7

Table 1. DLN2.6+ Mode Staging Diagram

MODE OPERATIONAL

RANGE

CIRCUITS

FUELED

CIRCUITS

PURGED

Diffusion (P) Ignition to

TTRF=1500

D5 None below 95%

SpeedPM1+PM2+PM3

above 95% Speed

Sub Piloted Premix

(L)

95% speed to

TTRF=2000

D5 + PM1 PM3 + PM2

Piloted Premix

(H)

TTRF=1500 to 2300 D5 + PM1 + PM3 PM2

Sub Premix (M) TTRF=1900 to 2300 PM1 + PM3 D5 + PM2

Premix (B) TTRF=2150 to BaseLoad

PM1 + PM3 + PM2 D5

Extended Piloted

Premix (A)

TTRF=2150 to Base

Load

D5 + PM1 + PM3 +

PM2

None

Load Rejection (I) Transient from

Premix or Sub-Premix

Modes

PM1 Premix: D5

Sub-Premix:

D5+PM2

Load Rejection (P or

L)

Transient from all

other Modes

D5 or D5 + PM1 (see

Note 1)

None

NOTES

1. Load Rejection to normal Full Speed No Load (FSNL) mode, which will be

either Diffusion Mode (D5 fueled) or Sub Piloted Premix Mode (D5 + PM1

fueled).

2. The unit will be load limited in each mode (except Premix and Ext. Piloted

Premix) at the upper end of the design ranges defined in this table.

B. Pre-start Conditions

Prior to unit start, the following start conditions must be satisfied for the Gas Fuel and Gas Purge

System.

1. Gas supply pressure within limits.

2. Gas module ventilation pressure normal.

3. Servo currents within limits.

4. P2 pressure feedback signal within limits.

5. Control valves track commands within limits.

6. Valve position feedback within limits.

7. All GCV’s closed.




8

8. Auxiliary Stop Valve and SRV cavity pressure normal.

9. No gas purge valve position faults.

During the start period the above conditions are continuously checked by the control system If any

condition is not satisfied the unit will not be allowed to start.

Prior to starting the gas fuel system, the following systems must be operating:

• Gas fuel compressors, when pipe line supply pressure is insufficient

• Instrument air system

Pre-Ignition P2 Pressure High Start Inhibit

The Pre-Ignition P2 Pressure High protection sequencing makes sure that the P2 cavity is clear of any pressure with the vent valve open, just prior to light-off. If the P2 pressure (FPG2) exceeds a

specified increasing pressure for a determined amount of seconds prior to firing permissive (L2TVZ),

a pre-ignition trip or Start Inhibit (L4PRETX) and alarm will occur.

C. Startup and Loading Operation

Gas Purge Valve Test

At the very beginning of startup the gas purge valves are tested automatically by the controls to verify

that slew times are within specification. All valves are cycled open and closed with the resultant time

to complete each operation for each valve being measured. An alarm signal is generated for any valve

that does not cycle within the allotted time frame. Alarm signals are generated for both open and

close time violations for each valve. The turbine will not start cranking if any valve fails to pass the

test. Once the gas purge valve test is complete, the gas turbine starts to crank.

Gas Fuel Leak Test

The Gas Leak Test will test the SRV and GCV’s for high leakage rates upon startup and shutdown by

monitoring the pressure in the P2 cavity. The tests will take place when the turbine starts purging

speed for the startup test and just after the turbine is shutdown for the shutdown test. Once either of

these enable commands have been met, the test will start. The Gas Leak Test is composed of four (4)

steps. Refer to Figure 2 Gas Leak Test Function Block Diagram.

Test A:

Test A will monitor the leakage across the SRV. The Auxiliary Stop Valve will be commanded open,

the Fuel Gas Vent Solenoid valve will be commanded closed (20VG-1), and the Fuel Gas Trip

Solenoid will be opened (20FG-1) to allow the trip system piping to fill up with oil for the next phase

of testing. If the leakage across the SRV is excessive and the P2 cavity pressure rises aboveK86GLTA pressure in K86GLT1 seconds, turbine startup will be inhibited and the machine will

shutdown.

Open SRV:

Once Test A has been passed, the SRV will be commanded open by forcing the SRV

pressure offset to a large pressure value for K86GLT2 seconds to ensure that the P2 cavity

has been pressurized to full line (supply) pressure. The SRV and the Aux Stop Valve will




9

then be commanded closed again and Test B will start. When K86GLT2 times out, the P1 pressure and P2 pressure will be latched and compared to each other. If the difference

between the two pressures is greater than K96FG_DIFF an alarm will notify the controller.

Test B:

Test B will monitor the P2 pressure and make sure that the GCV’s or Vent Valve is not leakingexcessively. If the P2 pressure drops below more than K86GLTB psi after K86GLT3 seconds, the

turbine will start inhibit and latch in an alarm. If Test B passes after K86GLT3 seconds, the vent will

open and the pressure will drain out of the P2 cavity. If there is no Aux Stop Valve, the test is over,

and the turbine will proceed with normal operation.

Relieve Pressure:

If the Aux Stop Valve is required for the system, the test will wait 5 seconds to ensure that all valves

have been returned to normal state. The SRV will then be opened again to relieve any pressure that

has been built up between that Aux Stop Valve and the SRV during the tests. Once this pressure has

been drained, the SRV will close and the test will be completed.

NOTE

If the SRV does not return to its normal sealed position when the test times out, the

P2 Pre-Ignition Trip will alarm and inhibit startup.




10

time

Test A Test B

K86GLT1 K86GLT2 K86GLT3

P1 pressure

Test B Limit

Test A Limit

P2 pressure with

vent open

Test A Fail

Test B Fail

P2 Pressure

vent valve

20VG-1aux stop vlv

VS4-1

SRV

GCV(s)

open

closedopen

closed

open

closed

open

closed

FG Trip

Solenoid

20FG-1

open

closed

K86GLT4

K86GLT5 K86GLT6

if Aux Stop Valve

Option

relieve

pressureopen SRV

Figure 2. Gas Fuel Leak Test Function Block Diagram




11

Valve Test Condition Aux

Stop Vlv

SRV Vent

Vlv

GCVs Pass Criteria Test Time Action

GSRV Unit Start and

Normal Shutdown *Open Closed Closed Closed P2 < 100 PSIG 30 sec

Startup

Lockout

FPG2

x-mitters

Unit Start and

Normal Shutdown *Open Open Closed Closed

ABS (P1-P2) <

20 PSI1 sec Alarm

GCV Unit Start and Normal Shutdown *

Closed Closed Closed ClosedP2 > P1 – 150

PSI30 sec

Startup

Lockout

Venting Unit Start and

Normal Shutdown *Closed Closed Open Closed P2 < 6 psi

After vent

timer

Startup

Lockout

Post Ignition P2 Pressure High and Low Trip

After firing and before warm-up complete, the P2 pressure is monitored to ensure that it is within a

predetermined pressure range. The desired unit P2 pressure during warm-up is calculated to achieve

the unit pressure range. If the pressure is lower or higher than a specified tolerance for a given time

frame, the unit will alarm and trip.

Start

The gas turbine is started in D5 Diffusion mode. When the unit reaches firing speed, the SRV (VSR-

1) and the D5 Diffusion GCV (VGC-1) servo controls are enabled. The P2 cavity vent valve (VA13-

15) closes. The SRV opens to control P2 pressure to its setpoint, and the D5 GCV (VGC-1) opens to

the ignition setpoint.

After flame is established and the turbine warm-up cycle is complete, the unit accelerates to Full

Speed No Load (FSNL) in Diffusion mode. At 95% speed, the unit transfers to Sub Piloted Premix

mode (SPPM). The control code also has the capability of delaying the Diffusion to SPPM transfer

up to TTRF1 levels of 1500 degrees F. At the SPPM transfer point, the PM1 GCV (VGC-2) is

enabled open to the Prefill position. After Prefill is complete, the PM1 GCV (VGC-2) is commanded

to a target fuel split. The purge system will be enabled 60 seconds after the unit reaches 95% speed.

Once the purge system is enabled in SPPM mode, the PM3 purge valves will open. After an 11

second delay, the PM2 purge valves will open, admitting compressor discharge air into the gas

manifolds. If Diffusion mode is utilized above 95% speed, the purge on sequence will be PM1, then

PM3, then PM2 (with 11 second delays).

At rated speed, the unit is synchronized and the breaker is closed. The unit load increases towards full

load. The fuel system remains in Sub Piloted Premix mode, until the first combustion reference

temperature (TTRF1) switch point. At this switch point, the fuel system transfers to Piloted Premix

(PPM) mode. This first switch point occurs between TTRF1=1500 and TTRF1=2000.

At the Piloted Premix (PPM) switch point, the PM3 circuit purge valves (VA13-3, VA13-4) are

commanded closed. When these valves are confirmed closed, the PM3 GCV (VGC-3) is enabled

open to the Prefill position. After Prefill is complete, the PM3 GCV (VGC-3) is commanded to atarget fuel split. The fuel split schedule for each DLN mode is a function of combustion reference

temperature (TTRF1). The unit is now in Piloted Premix (PPM) Steady State mode.

At the second combustion reference temperature (TTRF1) switch point, the fuel system transfers to

Sub Premix mode. This switch point occurs between TTRF1=1900 and TTRF1=2300. The D5 GCV

(VGC-1) is commanded closed and confirmed closed and the D5 circuit purge valves (VA13-1,

VA13-2) are commanded open, admitting compressor discharge air into the gas manifold. The unit is

now in Sub Premix (SPM) Steady State mode.




12

At the third combustion reference temperature (TTRF1) switch point, the fuel system transfers toPremix mode. This switch point occurs between TTRF1=2150 and TTRF1=2300. The PM2 circuit

purge valves (VA13-23, VA13-24)) are commanded closed. Once confirmed closed, the PM2 GCV

(VGC-4) is enabled open to the Prefill position. After Prefill is complete, the PM2 GCV (VGC-4) is

commanded to a target fuel split. The unit is now in Premix (PM) Steady State mode. The unit

continues to operate in Premix mode up to Base load.

A backup mode (Extended Piloted Premix) is also available for testing, commissioning and fault

accommodation. This mode provides fuel to all four fuel circuits. Ext.PPM mode can be enabled by a

pushbutton on the HMI screen. If this option is selected, the unit will load using SPPM, PPM, and

Ext.PPM modes, and will not use SPM or PM modes. Ext.PPM mode is also used if a D5 purge fault

occurs while in Premix mode (see section V).

The shutdown sequence of the fuel system is the reverse of the startup and otherwise similar. During

shutdown the unit transfers to SPM, PPM, SPPM and then to Diffusion mode, down to flame out at

approximately 17% speed. At approximately 94% turbine speed the gas purge valves are closed.

After flame out the hydraulic trip system is de-energized and the SRV and GCV’s close. The P2

cavity vent valve opens.

IV. CONTROL

A. P2 Pressure Control

The SRV (VSR-1) modulates to control gas supply pressure (P2) to the independent GCV’s. The

median selected value FPG2 of the triple redundant P2 pressure transducers (96FG-2A, 96FG-2B,

96FG-2C) is used in the Proportional + Integral controller. The P2 pressure setpoint (FPGR) is a

function of unit speed (TNH). The output of the proportional + integral controller is a valve position

reference (FRCROUT). The position command to the SRV is switched negative to ensure fast and

positive shutoff in the event of a unit trip, or following flame out on shutdown. This negative position

command, (FPKGSD) saturates the servo amplifier current in the closed direction. FRCROUT is the

position reference to a servo amplifier. Refer to Figure3 SRV P2 Pressure Control Algorithm

Diagram.




13

P2 Reference =

Speed(TNH) *

Gain (FPKGNG)

+ Offset(FPKGNO)

Speed

(TNH)

Proportional

+Integral

Control

GFPRGR

Application Code Control Algorithm

MED

Select

96FG-2A

96FG-2B

96FG-2C

Signal Space

Signal SpaceOutput

Signal SpaceInputs

FAGR

Inputs

FRCROUT

SRV Control

Fault LogicFSGR

FPG2

(Not Tracking)

Protection

Permissives

True

False

Shutdown

Command

L3GRV

FPKGSD

FRCROUT

P2 Pressure

Fault Logic

(Not Tracking)

Figure 3. SRV P2 Pressure Control Algorithm Diagram

The P2 pressure setpoint is lower at low speed, in order to provide better flow control at low fuelflow. Thus, the GCV does not have to reduce full line pressure, allowing the GCV to open further,

providing for better low flow control. The P2 pressure increases linearly to the 100% rated speed

pressure setpoint. The pressure downstream of the GCV’s (P3) at maximum fuel flow is used to

determine the rated speed pressure setpoint. The downstream pressure (P3) is a function of gas

turbine compressor pressure ratio, total flow, flow split settings and fuel nozzle size. The rated speed

pressure setpoint must be sufficient to maintain critical pressure drop across the GCV’s at max fuel

flow. The controlled P2 pressure and the critical pressure drop design of the gas control valves

ensures that the percentage valve stroke is proportional to percentage fuel flow.

The control of the SRV is accomplished by using an inner and outer loop. The SRV’s position control

loop is the inner control loop. The pressure control loop is the outer control loop. Three LVDT’s

sense SRV stem position and their outputs are returned to each channel <R,S,T> of the control

system. The error between the position command FRCROUT and the position feedback FSGR then

becomes the input to the servo amplifier. The servo amplifier drives the servo valve in the direction

required to decrease the position error. A null current bias is applied to the amplified signal in order

to overcome the fail safe servo spring bias. Refer to Figure 4 SRV Control Schematic.




14

f r o m

H y d O i l

V A I C

I O_

C A L

P r e s s =

f

( m a

)

V S V O

+ -

9 6 F G -

2 A

9 6 F G -

2 B

9 6 F G

-

2 C

p r e s s

m_

a m p s

< T > S

R V S e r v o

V a

l v e

9 0 S R

- 1

D / A

C u r r e n t_ G a i n

C u r r e n t_

B i a s ( N u l l )

f r o m A u x

S t o p

V S R - 1

9 6 S R - 2

9 6 S R - 1

t o G a

s C o n t r o l

V a l v e s

F R C R O U T

A p p l i c

a t i o n C o d e

C o n t r o l A l g o r i t h m

s e e f i g u r e 3

9 6 F G - 2

A

9 6 F G - 2

B

9 6 F G - 2

C

P 2 P r e s s u r e

F e e d b a c k

x 3

F i r m w a r e

T B A I

A / D

S i g n a l S p a c e

I n p u t s


O u t p u t

I O_

C A L

F S G R =

f

( V O L T S )

M e

d i a n

S e l e c

t e d

p o s i t i o n

V o

l t s + +


I n p u t s

A / D

F S G R

F S G R

< S >

F i r m w a r e

T S V O

< R >

V o t e d C u r r e n t - F A G R

F A G R

L V D T

P o s i t i o n

F e e

d b a c

k

x 3

D r a i n

f r o m

H y d T r i p O i l

V H 5 - 1

< R , S , T >

< R >

< S >

< T >

V S V O

V S V O

< S >

< T >

< R > <

S > <

T >

V A I C

V A I C

< S >

< T >

9 6 S R - 3

Figure 4. SRV Speed Ratio Valve Control Schematic




15

B. Gas Flow and Split Control

The gas control valves (VGC-1, VGC-2, VGC-3, VGC-4) modulate to control fuel flow and split

between the four fuel circuits. The percentage of fuel to each circuit is a function of Combustion

Reference Temperature, and DLN operating mode. The control system’s fuel command, FSR, is the

percentage of maximum fuel flow required by the control system to maintain either speed/load, or

another setpoint. The combined flow references used to position the four independent control valves

is proportional to FSR. FSR is divided into two parts, which make up the fuel split setpoint, FSR1

and FSR2. FSR1 is the percentage of maximum fuel flow required from the Liquid Fuel System, and

FSR2 is the percentage of maximum fuel flow required from the Gas Fuel System. FX1 is the fuel-

split command. A fuel-split command FX1 of 1.0 is equal to 100 % liquid fuel FSR1, and a fuel-split

command FX1 of 0.0 is equal to 100% gas fuel FSR2. For a gas only machine FSR2 is always equal

to FSR.

FSR2 is again divided into four parts for the four independent control valves. FQRG1, FQRG2,

FQRG3, and FQRG4 are the percentage of FSR2 to be sent to the D5 Diffusion gas fuel nozzles, the

PM1 Premix center gas fuel nozzles, PM3Premix outer gas fuel nozzles, and PM2 Premix outer gas

fuel nozzles. FSRG1OUT, FSRG2OUT, FSRG3OUT, and FSRG4OUT are the final output signals to

the position loop regulators after the shutdown position select logic. The position commands to each

valve are switched negative to ensure fast and positive shutoff in the event of a unit trip, or following

flame out on shutdown. This negative position command, FSKSHUT saturates the servo amplifier

current in the closed direction. FSRG1OUT is the position reference to a servo amplifier, which

drives the coils of the D5 Diffusion GCV. FSRG2OUT is the position reference to a servo amplifier,

which drives the coils of the PM1 Premix GCV. FSRG3OUT is the position reference to a servo

amplifier, which drives the coils of the PM3 Premix GCV. FSRG4OUT is the position reference to a

servo amplifier, which drives the coils of the PM2 Premix GCV. Refer to Figure 5 GCV Control

Algorithm Diagram.

FSR

MIN Select

Application Code Control Algorithm

Signal SpaceOutput

Signal SpaceInputs

FAGn

FSRGnOUT

GCV ControlFault Logic

FSGn

FSR

(Not Tracking)

Protection

Permissives

True

False

Shutdown

Command

L3GnX

FSKSHUT

FSRGnOUT

Rate

Limit

Combustion Ref Temp

(TTRF)

DLN Split ScheduleDLN Mode

Selection Logic

FX1

FSR2

+

-

Fuel Split Command

FSR1

Figure 5. GCV Control Algorithm Diagram




16

Three LVDT’s sense the GCV’s stem position and their outputs are returned to each channel of thecontrol system. The error between the position command and the position feedback then becomes the

input to the servo amplifiers. The servo amplifiers drive the servo valves in the direction required to

decrease the position error. A null current bias is applied to the amplified signal in order to overcome

the fail safe servo spring bias. Refer to Figure 6 GCV Control Schematic.

from

Hyd Oil

VSVO

+-

SRV Servo Valve

65GC-n

D/A

Current_GainCurrent_Bias (Null)

from SRV

VGC-n

96GC-m

96GC-n

to Gas

Manifolds

FSRGnOUT

Application Code

Control Algorithm

see figure 5

Signal SpaceOutput

IO_CAL

FSGn =

f (VOLTS)

Median Selected

p

o

s

i

t

i

o

n Volts

+

+

Signal SpaceInputs

A/D

FSGn

FSGn

<S>

Firmware

TSVO

<R>

Voted Current - FAGn

FAGn

LVDT

Position

Feedback

x3

<R>

<S><T>

VSVOVSVO

<S>

<T>

<T>

96GC-o

VGC-n VGC-1 VGC-2 VGC-3 VGC-4

65GC-n 65GC-1 65GC-2 65GC-3 65GC-496GC-m 96GC-1 96GC-4 96GC-7 96GC-10

96GC-n 96GC-2 96GC-5 96GC-8 96GC-11

96GC-o 96GC-3 96GC-6 96GC-9 96GC-12

FSG-n FSG-1 FSG-2 FSG-3 FSG-4

FAG-n FAG-1 FAG-2 FAG-3 FAG-4

FSRGnOUT FSRG1OUT FSRG2OUT FSRG3OUT FSRG4OUT

GCV Insturmentation and Control Signals

<R>

Figure 6. GCV Control Schematic

C. Gas Fuel Temperature Compensation

Redundant gas temperature thermocouples (FTG-1A, -1B, -2A, -2B) measure supply temperature at

the auxiliary stop valve inlet. The median selected value is used in the controls to compensate for

varying gas supply. In order to maintain consistent fuel flow during open loop fuel control, acorrection factor is applied to the control valves fuel command. This correction factor is a function of

the fuel temperature deviation from nominal design temperature. All open loop fuel control setpoints

are set for the nominal design temperature. The correction factor increases or decreases the fuel

stroke command in order to provide the target mass flow with changing fuel temperature.




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D. Gas Purge Control

Purge air flow is controlled by purge valves (VA13-nn) arranged in a double block and bleed

configuration. The purge system is designed in an independent flow path scheme. The diffusion

purge flow path is controlled by two purge valves (VA13-1 and VA13-2). The PM1 purge flow path

is controlled by two valves (VA13-20 and VA13-21). The PM3 purge flow path is controlled by two

valves (VA13-3 and VA13-4). The PM2 purge flow path is controlled by two valves (VA13-23 and

VA13-24). Solenoid valves (20PG-nn) pneumatically actuate the purge valves open or closed.

The purge valve actuation system is designed to control the rate at which fuel is purged into the

combustors. A manual needle valve provides a means of adjusting the opening rate for all of the

purge valves, except for VA13-2. An I/P controller is used for controlling the position of VA13-2

(gas-side Diffusion purge valve). The needle valves for the air-side purge valves are set to open the

purge valves quickly, and the needle valves for the gas-side purge valves are set to open the purgevalves at a slower rate. The I/P controller on VA13-2 provides additional flexibility for valve opening

rates and allows for steady state positioning at mid-stroke levels. A quick exhaust valve (VA36-nn) is

utilized for rapid closure of the purge valves. Refer to gas fuel purge schematic (MLI-0477) for

settings. The control system energizes the solenoid valves in order to open the purge valves. When a

solenoid valve is energized, pilot air is supplied to the quick exhaust valve (VA36-nn) which opens,

allowing air to flow to the purge valve (VA13-nn) actuator. The purge valves open at a rate set by the

metering needle valve (or VA13-2 I/P controller) in the air supply line of the VA36 valve, and allow

purge air to flow to the gas manifold. A purge valve test is performed automatically by the controls at

the beginning of machine startup to verify that valve open/close slew times are within specification.

Refer to Startup and Shutdown Control & Protection Article for further information.

The purge valves provide different functions depending on the mode of operation. While operating in

purge mode, they provide the flow control to the gas manifolds. While operating in blocking mode

these valves provide a double block and bleed isolation of fuel from the purge system. An inter-cavity

vent valve (20VG-nn) is located between the two purge valves which provides a block and bleed

system. In the event that fuel leaks past the gas purge valves in either direction and becomes too

excessive for the vent valve to bleed off, pressure switches will sense the cavity pressure, and an

appropriate action will be executed as described in Section V Protection.

V. PROTECTION

The following key describes the protective actions initiated by the control system for the gas fuel and gas

fuel purge systems.

Key: A = Alarm

SI = Start Inhibit

LO = Lockout Loading & DLN Mode Transfers

RB = Normal load runback until condition clears

FRB = Fast load runback until condition clears

PIT = Pre-ignition Trip (No Trip after Warmup complete)SD = Unit Shutdown

STP = Soft trip

TP = Trip

OFF = System is turned off

PBR = Push button reset




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A. P2 Pressure Control Protection and SRV Position Control Protection

Alarms and protective actions are initiated by the control system to protect the gas turbine and

combustion hardware from a loss of fuel control. The following table illustrates these protective

features.

In addition to P2 pressure protection, valve position fault protection is provided in the control system.If the SRV is not tracking position commands, it will result in DLN split errors, loss of load control,

and potential trips. The SRV has triple redundant position feedback transducers (LVDT’s).

Table 2. Protective levels and actions for the Gas Fuel System – Gas Pressure

Test Action Description

SRV Not

TrackingA, TP, PBR

Alarm if the SRV valve actual position is not following

specified position reference until warm-up is complete.

Trip if the SRV valve actual position is not followingspecified position reference until warm-up is complete.

Pre-Ignition

P2 Check A, SI, TP, PBR

If the P2 pressure exceeds specifications for

predetermined seconds before firing permissive isreached.

Post Ignition

P2 CheckA, TP, PBR

If the P2 Pressure exceeds specifications for

predetermined seconds between firing permissive and

warmup complete, alarm and trip the unit. If the P2

pressure falls below specified pressure for

predetermined seconds between firing permissive and

warmup complete, alarm and trip the unit.

Running P2Check

A, RB

If the P2 pressure falls below specified limit for

predetermined seconds, alarm and run the unit back to

safe mode.

Bottle Test A, SI, TP, PBR

If the P2 pressure rises above specified pressure during

test A, start inhibit. If the P2 pressure dips belowspecified pressure during test B, start inhibit. If the P1

pressure and P2 pressure transmitters do not read within

specified differential pressure of each other, alarm the

controller.

Low fuel

pressureA, SI, RM

Low fuel module inlet pressure (P1) or gas control valve

inlet pressure (P2) results in an alarm and unload to

SPPM mode for gas-only units. Dual fuel units willalarm and initiate a transfer to liquid fuel operation.

B. Fuel Temperature and Modified Wobbe Index Protection

In order to protect the gas fuel system hardware, triple sensors at the gas module inlet monitor fuel

temperature. High fuel temperature readings will result in the following actions:

• High temperature: Alarm

• High-High temperature: Shutdown performance heater

• High-High-High temperature: Shutdown turbine

In order to protect against combustor damage, the unit is not allowed to operate above a specified

TTRF1 level (currently set at 1900 F) if fuel properties are outside of the design Modified Wobbe




19

Index (MWI) by more than 5%. MWI is calculated based on site-specific fuel properties and fueltemperature. If the control detects MWI is outside of limits, operation will be limited to the specified

TTRF1 level. If the unit is already operating above the specified TTRF1 level when the MWI fault

occurs, the control system will unload the unit.

C. Independent Flow Path Purge Protection

Alarms and protective actions are initiated by the control system to protect the purge system and

combustion hardware from loss of purge or failure to block gas fuel from entering the purge system.Failure to purge fuel from unused fuel circuits poses a risk of auto-ignition, formation of condensate

in the piping or fuel nozzle back flow. A loss of fuel blocking can result in a hazardous condition

where gas fuel is present in the purge system, with the potential for fuel ignition. Multiple sensors are

used in the purge fault detection strategy, including open and closed limit switches on each valve, as

well as purge cavity pressure switches. A low pressure while purging indicates a loss of purge faultand a high pressure while blocking indicates a loss of blocking fault. The controls utilizes all of these

sensors in the protective strategy in order to provide single fault tolerant protection. The following

table illustrates these protective actions. Refer to Table 7 Independent Flow Path Purge Protection for

more information.




20

Table 3. Independent Flow Path Purge Protection

EVENT FAULT SETPOINT ACTIONLoss of Purge

(LOP) • Single limit switch

indicating out of position

or a single pressure switch

indicating low

< 50 psig

Alarm

• Both limit switches out of

position on a single valve

(double fault) OR

• Voted low pressure OR

• Combination of two: valve

limit switches or limit

switch & pressure fault

< 50 psig • Alarm.

• Lockout Purge to faulted circuit.

• D5 Purge Fault: Lockout PM and

SPM modes (immediate transfer

to Ext.PPM or PPM)

• PM1, PM3 or PM2 Fault: Lockout

mode transfers to DLN modes that

will add fuel to faulted circuit if

the fault persists for 15 minutes.Low D5 Purge

Air

Temperature

• D5 temperature sensors

(median-select) indicate

low temperature with D5 purge commanded on.

< 270 deg. F for 10

minutes. Alarm

• D5 temperature sensors

(median-select) indicatelow temperature with D5

purge commanded on.

< 240 deg. F for 15minutes

• Alarm.

• Lockout D5 purge.

• Lockout PM and and SPM modes

(immediate transfer to Ext.PPM or

PPM)

Loss of

Blocking

(LOB)

• Single limit switch

indicating out of position

or a single pressure switch

indicating high

> 50 psig

Alarm, Pre-Ignition Trip.

•

Limit switch double faulton a single valve OR

• Combination of two: valve

limit switches or limit

switch & pressure fault

> 50 psig Shutdown

• Voted high pressure OR

• Both limit switches on a

single valve out of position

plus any other single valve

limit switch or pressure

fault OR

• Three single faults in a row

(VA13-1 limit switch fault; pressure switch fault;

VA13-2 limit switch fault)

> 50 psig Trip

Low purge

pressure ratio

• Low purge pressure sensed

at manifold

< 0.94 Ratio Alarm

Consult the Control Specification for detailed adjustments and settings of the gas fuel and gas fuel

purge systems.

Consult Device Summary for detailed device settings and calibration.








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GEK111712 Control & Protection